1. 1. It has been shown that gamma radiation administered to the sporogonous cycle of the malarial parasite Plasmodium gallinaceum inhibits its normal development within the host mosquito Aedes aegypti. The morphological changes which occur in the parasite in vivo, as a consequence of the exposure to radiation, have been described in some detail, and are demonstrated in part in the photographs which accompany the text. 2. 2. A certain proportion of the parasites survive exposure to 5,000, 10,000, and 15,000 r and complete their normal development to produce viable, infective sporozoites even though the rate of development may be retarded to a lesser or greater extent. None of the parasites, however, survives exposure to 20,000 r or more. 3. 3. Regardless of the amount of radiation that has been administered, vacuole formation around the k-granule of the parasite appears uniformly as the first, and for a time, as the only specific morphological effect resulting from the exposure to radiation. 4. 4. The Brownian-like motion of the parasite granules which typically occurs in degenerating drug-treated parasites has been found to occur also in parasites exposed to gamma radiation.